In the field of printing inks, coating materials, and the like, which use oxidative polymerization-type resins as constituent components, a dryer is added as a curing accelerator for drying the resins. Dryers generally used for the resins include metal salts (may be abbreviated as “metal soaps” hereinafter) of heavy metals, such as cobalt, manganese, lead, iron, zinc, and the like, with various carboxylic acids.
Coating materials containing oxidative polymerization-type resins are usually applied outdoor, and thus it is desired to enhance working efficiency of outdoor coating by curing the coating materials within a short time at outside air temperature from a low temperature to room temperature throughout one year. Therefore, organic acid metal salts of cobalt, manganese, and the like, which have relatively high curing acceleratory, are mainly used as a curing accelerator (dryer) for oxidative polymerization-type resins.
Cobalt metal soaps have excellent drying performance, but when they are used in a large amount for shortening a drying time, surface drying of printing inks and coating materials very rapidly proceeds, and thus the problem of wrinkling and shrinkage due to skinning occurs, thereby causing difficulty in satisfying both the short drying time and the prevention of wrinkling and shrinkage due to skinning.
In addition, from the viewpoints that cobalt compounds used as raw materials of cobalt metal soaps are concerned about carcinogenicity because they are listed in Group 2B “Possibly carcinogenic to humans” in the list of carcinogenic risks of the International Agency for Research on Cancer and that cobalt metal soaps highly cost because metallic cobalt is a rare metal and thus suffers from unstable supply, there is demand for a curing accelerator having high curing performance using the cobalt metal soap in a smaller amount.
Therefore, as a drying accelerator having high drying performance using a smaller amount of the cobalt metal soap, a drying accelerator containing a cobalt metal soap, a manganese metal soap, and at least one aminoalcohol selected from diethanolamine, diethylethanolamine, dibutylethanolamine, and n-butyldiethanolamine has been proposed (refer to, for example, Patent Literature 1). However, the cobalt metal soap is still used, and thus the problem of concern about carcinogenicity, unstable supply of the raw material, and high cost cannot be resolved.
In addition, a drying accelerator (curing accelerator) using manganese soap in combination with bipyridyl has been proposed, prior to disclosure of Patent Literature 1, as a method for further shortening the drying time while preventing wrinkling and shrinkage due to skinning without using cobalt metal (refer to for example Patent Literature 2). However, the drying accelerator described in Patent Literature 2 has the advantage of not using a cobalt metal soap, but has the problem of increasing the drying time when used as a curing accelerator for a printing ink and a coating material.